Sigma 18-35 1.8 Canon compatibility?

The Sigma 18-35 1.8 would be really useful for me, however I'm concerned about focus issues reported on some Canons. At the moment I'm using a 650D, but will eventually upgrade, possibly to a 70D for the auto-focus video.

There's been reports of terrible AF with some Canons, the 70D in particular. Is it just a matter of the occasional lens having problems with the 70D or is it all of them? Thanks.